The present invention relates to moisture-curing, polyurethane hot-melt adhesive compositions based on polyethers. Hitherto hot-melt materials including eva, polyester or polyamide have been used for fast bonding processes and automatic adhesive application requiring quick setting bonds but these have usually been applied at temperatures of about 150.degree.-200.degree. C.
For example, solvent-free reactive polyurethane hot-melt materials, such as are disclosed in German Specification DOS 2609266, solve the problem of high application temperatures by application of a low-viscosity hot melt at 100.degree. C. which sets by crystallisation. Such adhesive systems exhibit good thermolytic and hydrolytic stability when chain-extended and cross-linked by atmospheric moisture. However, bonds provided by these crystallising polyurethane prepolymers have low elasticity before curing resulting in low initial peel strength immediately after bonding and have a lower setting rate which depends on the crystallisation rate of the polyester used. Other reactive crystalline polyurethane hot melts, such as the adhesive compositions disclosed in German Specification DOS 3236313, do show a better elasticity in the uncured state but have an undesirably high application temperature (approaching those of conventional hot-melt materials) and exhibit pot life problems at application. It is accordingly an object of the present invention to provide quick-setting, hot-melt adhesive compositions comprising polyurethane pre-polymers which have improved flexibility immediately after bonding whilst being heat stable and resistant to hydrolytic and chemical attack after curing.
Our copending U.S. patent application Ser. No. 347124 filed May 4, 1989 now U.S. Pat. No. 4,999,407 describes and claims a quick setting, hot-melt polyurethane adhesive composition comprising a mixture of at least two amorphous polyurethane prepolymers, each polyurethane prepolymer providing a different glass transition point for said composition. Preferably the first polyurethane prepolymer has a glass transition point above room temperature and a second polyurethane prepolymer has a glass transition point below room temperature.
These products are fast setting with high elasticity, even before curing.
Polyethers such as polypropylene oxide, polyethylene oxide or polyoxybutylene have low glass transition temperatures and little if any crystallinity and thus do not seem to lend themselves to the preparation of hot-melt compositions.